i had this same problem with my Titanium PowerBook G4 550 (yes, i know its a whole different architecture (PPC) for the Macintosh, but it uses the exact same graphics chip and it *is* a cross-platform operating system).
to fix it, i compiled frame buffer support into my kernel (under console drivers in make menuconfig) and specifically radeon frame buffer support. then in my XF86Config, under "Device" i turned on "Option UseFBdev" and it cleared everything up.

hope this helps...
- mike_________________"Yes, you may refer to me as Super Genius."